1. Field of the Invention
The present invention relates in general to a technique and apparatus for reducing NOx emissions, and in particular to a new and useful method and apparatus for reducing nitrogen oxide emissions without increasing the presence of unburned carbon and without causing conditions in the furnace which increase corrosion.
2. Description of the Related Art
There are many papers and patents that describe the use of staged combustion for controlling NOx emissions during the combustion of pulverized coal.
In a tangentially fired furnace, pulverized coal and the required air for combustion are introduced at the furnace corners tangent to an imaginary circle in the middle of the furnace. Controlling the emissions of nitric oxides (NOx) from these furnaces is accomplished through the use of staging, that is, the introduction of some of the combustion air downstream of the fuel for the purpose of allowing nitrogenous compounds from the fuel to convert to molecular nitrogen rather than to nitric oxide (NOx). The combustion air that has been used to accomplish this is called overfire air since it is introduced above the main combustion zone. This method of controlling NOx emissions has been very effective. However, it results in increased levels of unburned carbon due to the inefficiency of the combustion process and corrosion of the furnace's heat absorption surfaces due to the reducing (lack of oxygen) atmospheres required for the control of NOx emissions.